The present invention relates generally to semiconductor processing technology, and more particularly to a method for forming a metal-oxide-semiconductor (MOS) device with reduced transient enhanced diffusion.
As the gate width of MOS device reduces along with the size of semiconductor technology, the channel length between its source and drain is shortened. The shortening in channel length has led to severe problems such as hot carrier effect, which can degrade device performance and cause device breakdown. To remedy such problems, alternative drain structures such as lightly doped drain (LDD) structures have been developed. LDD structures act as parasitic resistors and absorb some of the energy within a MOS device, thereby reducing maximum energy in the channel region. This reduction in energy reduces the generation of the hot electrons that can hinder the performance of the MOS device.
The LDD structure is typically formed by implanting ions into one or more predetermined areas in a semiconductor substrate. During the implantation process, defects and damage may be caused to the semiconductor substrate. If the defects and damage are not repaired, the boundaries of the various doped regions, such as the source/drain regions, LDD regions and pocket implant areas, may expand significantly due to an effect called transient enhanced diffusion (TED), which typically occurs when the semiconductor substrate is annealed above a certain temperature after the ion implantation. For example, when forming sidewall spacers, the semiconductor substrate is thermally treated at a temperature ranging from 600 to 800 degrees Celsius. This temperature induces the TED effect that causes the undesired boundary shifting to the doped regions. As a result, the parasitic capacitance among various junctions of the doped regions is increased, thereby degrading the performance of the MOS device.
In order to eliminate the TED effect, a high temperature annealing process known as a rapid thermal annealing process (RTA or RTP) is typically performed immediately after the formation of the LDD structures to repair the damages caused by the ion implementation. However, by applying the conventional RTP to repair implant damage and reduce the TED effect, a new problem with thermal diffusion is introduced. During the RTP, the semiconductor substrate is thermally treated at a temperature higher than 800 degrees Celsius. This high temperature will cause thermal diffusion, which increases the junction depth of the doped regions in the substrate. This increased junction depth will hinder the performance of the MOS device.
Desirable in the art of semiconductor processing technology are methods for forming LDD structures that can suppress the TED effect and reduce thermal diffusion to improve performance of the MOS device.